The KivuWatt power station in Rwanda's Lake Kivu harvests methane to provide power to an energy-starved country

Deep beneath the waters of Lake Kivu in Africa's Rift Valley lies enough gas to power neighbouring Rwanda for the next 50 years - or to poison the two million people who live along its shores. "Deep springs carry CO2-rich water from volcanoes to the bottom of the lake, where bacteria turns some of it into methane," explains Jarmo Gummerus, country manager for Rwanda at US energy firm ContourGlobal. "In ordinary lakes, this gas would disperse slowly into the atmosphere; Kivu is so deep, all the gas is trapped under pressure at the bottom."

ContourGlobal's KivuWatt power station, inaugurated in May, now extracts and burns this methane to provide 25mW of power to an energy-starved Rwanda. The company is working to expand operations to 100mW - over half as much the current energy capacity of the country's entire national grid.

Stored safely underwater, Lake Kivu's methane reserve represents Rwanda's best hope for an energy-secure future. However, if the lake's pressure balance destabilises, it could become the country's greatest natural disaster. Over the next few hundred years, Kivu's gas reserves will continue to accumulate, until a tipping point is reached and millions of tonnes of suffocating CO2 is released.

Destabilising events such as volcanic activity or earthquakes could trigger an eruption well before then. In 1986, at Cameroon's Lake Nyos - one of only two other lakes to share Kivu's unusual geochemistry - a landslide triggered just such an eruption, asphyxiating 1,700 people within a 25km radius. Kivu is 2,000 times greater in size.

Given the scale of the hazard, not interfering with Kivu's gas-water balance seems like a good idea. But, Gummerus says, inaction could be even riskier. "We re-pump the de-gassed liquid back down to 240 metres to ensure that the two layers are not disturbed," he explains. "But if nothing is done, the gas pressure will, eventually, exceed the water pressure. By reducing the concentration of gas we're actually making the lake safer."